Allocate an individual to 'specific skill' training for X amount of time. After that time they have that particular skill "boosted" for X amount of time or usages.
Example:
Send your fighter to train accuracy for 9 hours (or whatever, based on your training facility's level) and for 1 day, 3 hrs, 1 mission, or whatever, they have a random boost to their accuracy. They are unavailable during training and if pulled early they get no bonus.
Suggested skills:
melee
accuracy
reload
speed
vision
morale
dexterity